TERMINAL NODE: OUTFALL BOUNDARY CONDITION

TYPES OF OUTFALL BOUNDARY CONDITION #

One of the most important components in any network are the outfall boundary conditions. In MiTS3, you can have outfalls that are Free, Normal and Fix.

OUTFALL CONDITION	DEFINITION
Free	This condition allows water to exit the system freely, without any external constraints or specified conditions. It essentially acts as an open exit, with the water level determined by the downstream channel’s characteristics.
Normal	The “Normal” condition is based on the critical depth of flow, which is the depth at which the specific energy is at a minimum for a given discharge. Essentially, it adjusts the outfall condition based on the normal flow conditions in the system.
Fix	A terminal node where the water surface elevation (stage) is set to a constant, user-defined value for the duration of the simulation. It is primarily used to represent a constant tailwater condition, such as a receiving water body with a stable pool level, regardless of the discharge rate. This is useful for scenarios where there are known or targeted discharge rates or levels, perhaps due to regulatory requirements or specific design criteria.

In this article, we will emphasize on the fix outfall condition

A simple MiTS model was created consisting of a single catchment generating inflow to a conduit, which then discharges to a terminal node with a fixed outfall level. The model was analysed using Dynamic Wave routing.

File sample: Click here

Information

Major Design Storm ARI	: 50 years
Selected storm duration	: 60 mins
Ground Level (m)	: 30.00
Start Drain IL (m)	: 29.00
End Drain IL (m)	: 28.00
Fix Outfall Water Level (m)	: 28.50
Drain dimension (mm)	: 1200 x 900

FIX OUTFALL CONDITION ANALYSIS RESULT #

Constant Water Depth #

Result and Discussion #

Based on the simulation depicted above, there are three phases occurring:

1. Rainfall Event: 0 – 60 minutes

At the start of the simulation, the conduit water depth is 0.25m. This is expected, as it corresponds to the fixed water level imposed at the outfall. From 0 to 30 minutes, the flow in the conduit increases from 0 to approximately 1.50 m³/s. Water depth increases from about 0.25 m to 0.46 m. From 30 to 60 minutes (where the peak rainfall period is occurring), the drain flow and water depth remain approximately constant.

2. Catchment draining: 60 – 90 minutes

After the storm ends at 60 minutes, the flow gradually decreases. By around 90 minutes, the flow reaches almost 0 m³/s. This indicates that rain has stopped. Hence, no more runoff is entering the system and the catchment has completely drained.

3. After 90 minutes: Why is water depth still at 0.25 m even though rainfall has already stopped?

This is the important observation when we introduce a fixed outfall. The fixed outfall is imposing a downstream water level. In other words, there is no flow after 90 minutes, but the conduit is still submerged to a depth of 0.25 m due to the fixed tailwater level.

Think of a pipe discharging into a lake. After the rain stops and no more water is flowing through the drain/ pipe conduit, the lake level stays the same, so water remains in the downstream part of the pipe instead of draining out completely.  

Why Doesn’t the Link Water Depth Match the Outfall Node Depth?  #

Intuitively, since the drain end invert level is 28.00 m and the tailwater level is fixed at 28.50 m, you might expect the water depth at the downstream end of the drain to be 0.50 m. In other words, it seems logical that the water level in the drain should rise to match the specified tailwater level. So why does the water depth shown in the drain not appear to reflect this? 

Here’s why. In MiTS, the reported link water depth for a conduit is generally the average depth within the conduit, not the depth at the upstream node or downstream node. MiTS internally computes three depths for each conduit:

1. Upstream end depth (Y₁)
2. Downstream end depth (Y₂)
3. Midpoint depth (used for reporting and hydraulic calculations)

The reported conduit depth is essentially the midpoint depth, which is approximately:

Y_mid=(Y₁+Y₂)​​/2

        ​=(0.00+0.50)​​/2

        ​=0.25m

Backwater effect (Tailwater level is higher than Headwater level) #

 In a model where the tailwater level is higher than the drain invert level or pipe crown elevation, Dynamic Wave Routing can simulate backwater effects and flow reversal. When the downstream water level exceeds the upstream water level, theoretically the hydraulic grade line reverses (changes direction), causing the water to flow from downstream to upstream. 

 A model was created, consisting of storage facilities, and having outlet drain to be lower than the specified tailwater level. 

File sample: Click here

Information

Downstream Drain IL	: 27.00
Tailwater level	: 28.00
Head difference between Tailwater level and Outlet IL	: 1.00m

Result and Discussion #

At the start of the simulation, negative outlet flow is observed. This indicates reverse flow from the downstream system into the storage (upstream system). The backflow occurs due to the downstream tailwater level being higher than the outlet control level, resulting in a submerged outlet condition and a reversed hydraulic gradient.

Since the specified tailwater level (28.00 m) is 1.00 m higher than the outlet invert level (27.00 m), the downstream boundary exerts a greater hydraulic head than the storage. Under Dynamic Wave Routing, this condition is physically represented by flow entering the system through the outlet structure, producing a negative discharge value (the circled region). 

As the simulation progresses, runoff inflow enters the storage and causes the water level within the storage to rise. This gradually reduces the head difference between the downstream tailwater and the storage water level. During this period, the storage accumulates water because inflow exceeds the effective discharge through the outlet. 

Eventually, it can be observed that the outlet flow transitions from negative to positive. This indicates that the water level within the storage has risen sufficiently to overcome the downstream tailwater level, thereby restoring the normal flow direction through the outlet structure. At this point, the hydraulic gradient becomes positive (sloping from higher elevation to lower elevation), allowing water to discharge from the storage towards the downstream boundary. 

CONCLUSION #

Having a fixed outfall boundary condition maintains a constant downstream water level throughout the analysis, regardless of the discharge rate entering or leaving the system. Consequently, the downstream hydraulic grade line remains fixed, providing a stable tailwater condition for the drainage network.

In addition, Dynamic Wave Routing can accurately represent backwater effects caused by an elevated downstream water level. When the specified tailwater level exceeds the outlet invert elevation and the upstream water level, the hydraulic gradient reverses, resulting in flow moving from downstream to upstream. 

Overall, the simulation confirms that downstream boundary conditions can significantly influence upstream hydraulic behaviour. This demonstrates the importance of considering tailwater conditions when analysing drainage systems that discharge into rivers, ponds, tidal waters, or other water bodies where downstream water levels may control system performance.